cfg80211: use parallel_ops for genl

cfg80211: use parallel_ops for genl

Over time, we really need to get rid of all of our global locking.
One of the things needed is to use parallel_ops. This isn't really
the most important (RTNL is much more important) but OTOH we just
keep adding uses of genl_family_attrbuf() now. Use .parallel_ops to
disallow this.

Reviewed-By: Denis Kenzior <denkenz@gmail.com>
Link: https://lore.kernel.org/r/20190729143109.18683-1-johannes@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>